Hyperoxia is when the oxygen partial pressure exceeds 1.6 bar/ata (considered the maximum upper limit for recreational diving) or when a diver is exposed to elevated partial pressures of oxygen for a number of minutes exceeding those recommended in the universally used NOAA (National Oceanographic Atmospheric Administration) diving tables. (The RAID nitrox specialty teaches divers to use oxygen-enriched gases and covers this and related topics in detail.)
This exposure may lead to any number of the following symptoms:
- Visual disturbance
- Ringing in the ears
- Muscle twitching
The symptoms may be sudden or gradual. Generally, open circuit air divers will be unlikely to experience this. Your depth limits and your no decompression limit (NDL) will not allow you to build up a partial pressure of oxygen that could be harmful.
With 21% oxygen in your normal air mix, you would need to descend to around 8 atmospheres (ata) of pressure to take you over the partial pressure of oxygen limit. (21% = 0.21 ata partial pressure of oxygen at the surface)
- The following demonstrates the effects of depth on oxygen pressure.
- For a 40-meters/130-feet dive, you are now at 5 ata of pressure: 5 × 0.21 = 1.05 ata.
- For 60 meters/200 feet, you are now at 7 ata of pressure: 7 × 0.21 = 1.47 ata.
- These examples are way over the RAID limit for this course program but illustrate how the oxygen pressure increases with depth. You can stay safe by not exceeding these limits. The maximum recreational depth limit is 40 meters/130 feet for divers who have the appropriate training and experience.
It is, however, possible with nitrox-certified divers who are trained to use higher concentrations of oxygen in their breathing gas to approach closer to the limits where oxygen toxicity is a tangible problem. Specialized training is required for nitrox diving.
IMPORTANT: The oxygen partial pressure limit of 1.6 is reached or even passed if uncontrolled descents are not stopped.